The space available for an HF antenna at my location is very limited. The plot where I live is only 5 meters wide. If you want an HF antenna that covers the 20, 17, 15, 12, and 10-meter bands, you quickly end up with an antenna like a Cobweb. I ran into the 7-band AWK Cobweb and after a year in operation it was time for a personal review.
Omnidirectional fan dipole
The Cobweb is an invention of G3TXQ (sk) and a variant of G3TPW’s Cobwebb. The latter is in turn a variation of the classic Halo antenna developed and patented in the 1940s. The Cobweb consists of multiple open dipoles in a square shape, making it a horizontally polarized omnidirectional antenna. It is also called an omnidirectional fan dipole. The small footprint of only 2.8 x 2.8 meters makes the antenna particularly popular.
Fed with a 1:4 matching transformer
The Cobweb has an impedance of 12.5 Ohms and must be fed using a 1:4 matching transformer to match it with a 50 Ohm transceiver. Additionally, a 1:1 balun is needed to feed this symmetrical antenna with asymmetrical coaxial cable. This is to prevent common mode currents and achieve optimal radiation. All elements are fed from a single feedpoint.
Affordably priced 7-band Cobweb
After experimenting with homebuilt versions of the Cobwebb, Classic Halo, and the Cobweb, I decided to purchase a commercially built version. I found the affordably priced 7-band Cobweb from AWK Antennas in Poland on eBay. This antenna covered all bands from 20 to 4 meters. I didn’t need the 6 and 4-meter bands, but they were a nice bonus.
All elements are pre-tuned
A week after ordering, I received the antenna. It was well-packaged in a long cardboard box with an instruction manual. What pleasantly surprised me is that the antenna is ready for use. All elements are pre-tuned and already connected to the 1:4 impedance transformer. You only need to unfold the spreaders, secure them, and mount the antenna on your mast using the supplied U-clamps.
Pre-assembled and pre-tuned, straight out of the box. Saves you a lot of time.
Lightweight and small wind surface
The spreaders are made of relatively small diameter fiberglass poles and are attached to the mast mount with aluminium tubes. It may seem somewhat fragile, but it’s sturdy enough. Moreover, it makes the antenna very lightweight with a relatively small wind surface. This makes it possible to mount the AWK Cobweb on a simple mast. The mast attachment looks sturdy. The dipoles are made of 1.5mm2 (AWG18) copper wire with PVC insulation and are securely attached to the spreaders with clamps. The ends of the dipoles are held together with elastic bands.
Mounted onto the mast. On top of the mast is a VHF/UHF halo.
The wires hold in place with small brackets and the ends tied together wit elastic cord.
4 and 6-meter bands
One thing to note is that the elements for the 4 and 6-meter bands do not have the square shape of the Cobweb but are laid out more in a U-shape. This means that these elements do not radiate omnidirectional. The U-shaped design is also unlikely to have an optimal or efficient radiation pattern.
The 1:4 matching transformer
The plastic feeder box containing the 1:4 matching transformer feels somewhat fragile. Inside the box is a wrapped FT240 ferrite toroid, mix unknown (probably #61), with enameled copper wire windings. These are connected to two metal plates to which the elements are soldered. The soldering work is not perfect, but it does the job. The connector (UHF SO-239) is located at the bottom of the box, which is quite clever because it reduces the chances of rainwater migrating into the connector. The box also has two extra M5 screws to allow you to add additional elements ;like for 30 and 40 meter band.
(photographed by DF3FX/F1VEO Félix)
1:1 balun is missing
What this antenna lacks but is essential is a 1:1 balun (or common mode choke). Make sure you build or buy one and connect it directly between the 1:4 transformer and the coaxial cable. An ugly balun, made from coiled coax, is too narrow-banded and not effective enough for this antenna. I added a 1:1 balun (grey box in the picture) based on the DG0SA design.
The 1:1 balun in the grey box directly attached to the impedance matching box with a male/male UHF coupler.
Less than 30 minutes to assemble the antenna
Assembling the AWK Cobweb for operation is very easy. It took me less than 30 minutes to unfold the antenna and mount it on the mast. Remarkably, except for the 15 meter band, the SWR is close to 1:1 on all bands. This is quite remarkable for a 7-band antenna. I should note that the antenna needs to be a certain distance off the ground to obtain a reliable SWR measurement. On the 15 meter band, the SWR was only acceptable at the lower end of the band. I had to tune it with the element length. Besides SWR, resonance was excellent on all bands as well.
The AWK Cobweb in operation. Notice the two smallest elements for 4 and 6 meters are in fact U-shaped dipoles.
Narrow in terms of SWR and resonance
The bandwidth of the antenna from Poland is narrow in terms of SWR and resonance. On 20 meters, it’s approximately 200 kHz with an SWR < 2:1. On 15 meters, it’s about 250 kHz, and on 10 meters, it’s a little under 400 kHz. The SWR remains below 2:1 across the entire 12 and 17 meter bands. On 10 meters, the antenna is tuned to 28,500 kHz, which puts the CW portion of that band out of reach. The 6 and 4 meter bands offer a bandwidth of at least 500 kHz. For HF, you will need a tuner to cover the entire bands.
Compared to a monoband Classic Halo
For a fair comparison, I initially placed the AWK on 6, 10, 15, and 20 meters against a monoband Classic Halo. The Classic Halo is a variant of the Cobweb but with folded dipoles. By transmitting simultaneously with both antennas, I was able to make a fair comparison in WSPR and FT8. It struck me that the Classic Halo performed better, especially on 10 and 15 meters. On average, it had a 2 dB better SNR, sometimes even up to 5 dB better on 10 meters. On 20 meters, the difference was marginal. These measurements were conducted over a period of three months.
I applied the same approach to reception, with both antennas connected to an SDRplay RSPduo that has two identical receivers. To my surprise, the AWK performed better than the Classic Halo in terms of reception. The results were opposite now. The AWK had an average SNR that was 3 dB better, sometimes up to 5 dB better on 10 meters. What was remarkable is that I could clearly hear a difference between the two antennas in SSB as well. The AWK produced less stronger signals but proved to be much less susceptible to local interference than the Classic Halo, especially man-made noise, which is prevalent in my location. These measurements were also conducted over a three-month period.
Compared to a stretched dipole
As a final test, I also compared the AWK on 10 meters with a half-wave stretched open dipole. When transmitting, the dipole was superior in its optimal direction, sometimes up to 8-10 dB better. Even in the least favorable direction, the dipole still proved to be slightly better. Next, I placed the monoband Classic Halo opposite the dipole. The omnidirectional characteristic of the Classic Halo meant that the dipole had to yield in its least favorable direction. What was further noticeable is that the straight dipole has significantly more (twice) bandwidth compared to both the Cobweb and the Classic Halo.
Performance on 6 meters
I also tested the 6 meter section of the antenna compared to a Classic Halo. The performance of the AWK on 6 meters is disappointing. Both in transmitting and receiving, a monoband Classic Halo is on average 6 dB better.
I have no direct indication for the poor transmitting performance of the Cobweb on the higher bands. However, a possible cause could be that the shape of the 10 and 6 meter elements leads to inefficient radiation. It is more likely that there is significant interaction between the various elements during transmission, resulting in disturbed radiation.
Mechanically, the AWK is an easy-to-install and robust antenna that has weathered some storms here. I was pleasantly surprised by its SNR performance. If the antenna performs this well in a noisy environment like mine, then I have high hopes it will work well everywhere. However, in terms of transmitting, the AWK simply does not perform well on 15 and 10 meter. Although I didn’t extensively test it on the 17 meter WARC band, it seems that the AWK performs as well in transmitting there as it does on 20 meter. The 4 and 6 meter band sections are a nice bonus but shouldn’t be taken too seriously.
To buy or not to buy?
The reception performance in terms of SNR can hardly be improved for an antenna. That’s not necessary with the AWK. However, yes, its transmitting performance leaves something to be desired. Fortunately, this can be easily improved by just increasing the power. If you have no expectations for the 4 and 6 meter band sections and have a tuner to stretch the bandwidth a bit, then yes, the AWK is a good purchase.